COMMUNICATION BY INDUCTION Filed June 17, 1958 2 Sheets-Sheet 2 ATTQQNEYS lUnited States TatentV Office 3,043,9l6 Patented July 10, 19562 Claims priority, application Great Britain .lune 28, 1957 1 Claim. (Cl. 179-82) In systems working by magnetic induction yfor selectively calling one of a number of persons, it is desirable and the phase and amplification from them adjusted and fed into the receiver in parallel or series, with the signalfrom the loop, so that the field due to interference arising at adistance is neutralized. By suitably placing the pickup devices, interference from sources in close proximity may also be neutralised. Where it is not convenient to J place pick-up coils outside the loop, a small neutralizing that the person called should be able to indicate that he I,

(or she) has received the call, and sometimes transmit further information by code.

Where speech is used and transmitted from the central system, it is also desirable that the person called shall be able to speak back. In order to achieve intelligible speech, however, it is necessary that the signal transmitter should have a higher level than is required for acknowledgement, or a simple code, and in some cases the interference will -be such as to render the speech impossible, or unreliable, where acknowledgement itself is satisfactory. Further, in many cases, the interference background will be subject to variations and for this reason it is an advantage to have provision for acknowledgement with a high degree of discrimination against inteference, in addition to speech. Fur-ther, the acknowledgement signal can be used to warn the operator, putting through a selective call, that the callee is about to speak.

The invention accordingly provides apparatus for communication by magnetic induction, wherein a pocket transmitter in, or near a loop can be caused to communicate with a caller by generating an alternating magnetic field of audio, or low radio frequency, by means of a small portable device, such signal being received in the loop system arranged to energise a visual, audio or other indicator. The frequency of the acknowledgement signal may be of the same order as that of the call signal and may be spaced between two call signal frequencies. Where speech is employed, however, it is desirable that the carrier frequency should be at least l5 kc., and may often be considerably higher than the highest call signal. In this case the acknowledgement signal frequency may conveniently be the same as the speech carrier frequency, or fall within the side bands of the speech modulated carrier.V Consideration will, however, have to be given to the distribution of'local electromagnetic interference in selecting the frequency used for acknowledgement.

The loop may be the same as that used for transmitting calls, although in many cases it will be necessary to take precautions in its layout to reduce the picking-up of eX- ternal interference, that will -be necessary if it were used loop, in series with the main loop, can be placed in position to neutralise the field of the main loop at the pick-up coil. Y

In many cases, however, after these steps are taken the interference will still be substantial, at least l,so far as concerns interference with the transmission of normal carrier speech. At a higher level the interference will make all forms of speech transmission impossible. It may, however, still be possible to send out acknowledgment signals, which can be received from the loop, either by the use of an exceedingly selective receiver, tuned to the transmission frequency, or by transmitting signals which'are modulated with a fixed frequency and having tuned circuits coupled to the loop, which areselective both to the carrier frequency and the modulation frequency. Further discrimination against interference can be obtained by causing the acknowledgment receiver to be operated The acknowledgment transmitter unit must be capable l of giving as high a power as possible in a small size. Where speech is to be employed, a transistor oscillator, coupled to a tuned ferrite rod aerial, may be modulated by a transistor amplifier and the same transistor amplifier can be caused to oscillate at a steady frequency by means of a switch, bringing in a tuned feedback circuit in place of a microphone for the purpose of acknowledgment. This apparatus may be powered by one or more small primary or secondary cells, which may be the same cells as are used to power the receiver. The receiver and the acknowledgment transmitter will not be required to Work at the same time. Where, however, it is necessary, a higher voltage may be employed for the acknowledgment device. As such device will normally be on only for a second orso, occasionally it is economical to employ quite low-capacity cells, provided they are capable of giving quite heavy instantaneous currents.

Considerable improvement may be obtained in the effective speech-to-interference level, with a given power consumption of the transmitter, by employing emphasized employed for almost complete side-band suppression in interference and it is necessary to reduce this as far as communication equipment are usually too elaborate' for the small'transmitter and methods of response of a nature of compromise have to be resorted to. One sideband (for speech) corresponding to audio frequencies of between 800 and 2,400, may advantageously be sent out at about twice, or more, .the level of the carrier. The second side-band can be vestigial. The receiver connected tothe loop will then be tuned either to have a band-pass extending from the carrier to the transmitted side-bands, corresponding to around 2,000 to 3,000 cycles or, with better advantage, to have two peaks, one of which can be very sharp, corresponding to the carrier frequency, and the other one broader and lower, with its centre corresponding to the peak of the side-bands transmitted.

Even further discrimination against some forms of interference may be effected by having the incoming signals,

either immediately or after amplification, with only a moderate `degree of selectivity, such as above described, divided into two channels, one of these channels terminating in a limiter stage followed by a highly-selective filter circuit tuned to the carrier frequency, the other channel, without a limiter circuit, feeding a band-pass filter tuned to the side-band transmitted. The signals from these two iilters are then mixed and applied to the rectier.

In general, where speech is employed there will be a substantial background noise heard in the receiver when no speech is being transmitted. While according to known practice the receiver may be designed so as to be quiescent until a carrier of an adequate level is received, this will not -allow the reception of speech in the most severe conditions. Ine one form of this invention the speech reception channel is normally switched on and the receipt of an acknowledgment signal switches it on. it then remains switched on either until switched off manually, or is maintained switchedon by the presence of the carrier signal and is switched off on the interruption of this signal for ya period.

Where the emphasized single side-band transmission is employed for speech, acknowledgment can be effected by modulating the carrier with a speech frequency, and the receiverattached to the loop may contain in the path operating the acknowledgment indicator a filter, sharply tuned either to the frequency of the side-band produced by. this note, or to the note itself. v

The portable transmitter for producing the emphasized side-band signal may consist ofay transistor oscillator modulated by the speech signal, which oscillator is coupled through a filter, which partially eliminates the carrier, to -anioutput stagevcoupled to a ferrite core tuned to `the middle kof the side-band to be emphasized.

' Where speech is not required, a simple form of the invention may be used, in which a resonance circuit is ex-A cited byV a pulse of current. resonance circuit consists of a ferrite-cored coil, with a low loss tuning condenser connected across it. A source of electromotive force, for example, a small accumulator, is also connected across the coil for a fraction of a second, causing a current to buildup in the inductance. When -t-he battery circuit is broken, the power in the inductance is expended in a damped oscillation in the tuned circuit.

In a modified arrangement, the current is caused to liow through a small part onlyvof the total tuned coil. This enables a larger tuned coil to be used and tuned with a small adjustable condenser.

The battery contact may be attached to a mechanically tuned vibrator, such as a tuning fork or reed, which prefstructions of portable transmitter.

Referring first to FlG.V l, loopX `feeds a selective ampliier A which is tuned to the carrier frequency. The signal from A, which is *fed* to a rectifier B and thence to a selective filter C, is detected by a unit D, to operate y the indicating lampy E. This lamp may be replaced by a sound-producingy device, or by a mechanical indicator. An anti-interference loop Y -feeds an interference signal through a phase and amplitude-adjusting network F and amplifier G, into the inputtoV A, in parallel or in series with Iloop X, to neutraliseexternal interference.

FIG. 2 shows a modification of the arrangement illustrated in FIG. l. This enables a higher degree of se- In one arrangement thisY FIGS. 3-7 are circuit diagrams showing various con- 4 lectivity to be obtained with the acknowledgment signals and also provides for `the reception of emphasised sideyband speech signals. Here -the loop H feeds a selective amplifier K, which is broadly tuned to the carrier frequency. From this filter part of the signal passes to a limiter L and a very selective filter M which is tuned to the carrier frequency. Part of the signal from K is also passed without being limited to a filter N, which is tuned to the side-band frequency of the speech, which includes the sideband 4frequency of the acknowledgment note. The outputs from M and N are mixed and fed to a rectifier R. The rectified signal from R is fed to a limiter stage Pand then to a very selective low-frequency fil-ter Q, which serves to operate an acknowledgment lamp S and also a switching device V. When the switching device V is operated, the output from the rectier R also passes to an amplifier T to operate a loudspeaker U. In the `arrangement shown, an anti-interference loop Y has its output also fed into K through phase and amplitude adjusting network F and amplifier G,- as in FIG. l. The arrangement of FIG. 2 may, of course, be used for the more selective reception o-f an acknowledgment signal, without the provision of speech.

Referring now to the acknowledgment 'transmitter illustrated in FIG. 3, a transistor T1 oscillates at the frequency yof a ferrite rod aerial L1 and tuning condenser C1 by virtue of a feedback winding L21 connected between the base and the emitter of T1 through a condenser C3, which lserves as a by-pass for the carrier frequency, but not for the modulation frequency employed, negative bias being supplied to the base of T1 through resistors R7 and R3. The circuit is tuned to give the carrier frequency required.

A transistor kT2 oscillates at the modulation frequency determined by a transformer L2 and a "condenser C2, a feedback winding L22 of the transformer being connected between base and emitter through a by-pass condenser C4, negative -bias being supplied to the base through a resistor R6. Signals of this frequency are fed back through a condenser C5 and the resistor R3 to modulate the supply of current through R3 and the secondary of L1 to the base of transistor T1 and hence modulate the oscillation of T1. Instead of employing variable condensers C1 and C2, these condensers may be fixed yand the inductances L1 and L2 adjusted by permeability tuning.

FIG. 4 illustrates a small portable transmitter for transmitting speech with emphasized side-band, in addition to the acknowledgment signals. The speech signal `from a microphone M is fed between the base and emitter of a transistor T3 through the by-pass condenser C6, bias reaching the base through a resistor R8. The output from the collector of T3 is fed` through a condenser C7 and a resistor R10 -to the base of a transistor T4, which receives bias through a resistor R11. A transistor T5 oscillates :at the carrier frequency by virtue of an inductance L5, tuned with a condenser C8 and having a feedback winding coupled between the base and emitter of T5 through a bypass `condenser C4, the -bias being fed to the base of TS through a resistor R12. Oscillations from this transistor (T5) are fed through the condenser C9 to the b-ase of T4 and are modulated by the audio frequency signals, applied to the base through a resistor R10. The output fromT4 is coupled to a transistorT through a transformer L3, which is tuned with a condenserClO. This transformer is tuned to the side-band to be passed but suiciently broadly to allow through a certain Aamount of the carrier. Bias for the base of T6 is supplied through a resistor R13 to a by-pass condenser C11. The collector circuit of T6 contains a ferrite-cored aerial L4 tuned to the side-band frequency to be transmitted by means of a condenser C12.

' The switch Z, comprising a change-over and a makecontact, serves to change over from Speech to Call by switching the base of the transistor T3 fromthe micro phone to a secondary on a transformer L6 and at the same time connecting a tuning condenser C13 across the primary of this transformer. A switch J serves to switch on the transmitter. Both I and Z may lbe ringer-operated press buttons.

FIG. 5 shows `an alternative method of coupling the modulated oscillator to the output stage so as to transmit predominantly single side-band. A transistor T7 oscillates at a frequency determined by a tuned transformer L7 and condenser C14. Feedback from the secondary of the tuned transformer is applied between the base and emitter of T7 through a signal :frequency by-pass condenser C15. Audio-frequency signals to modulate the oscillation are fed in through a condenser C16. The modulated carrier frequency output from T7 is fed through a condenser C17 to the base of an output transistor T8. In the collector circuit of this transistor is a ferrite rod aerial L8 tuned by a condenser C18 to the side-band frequency. A tuned circuit comprising an inductor L9, tuned by a condenser C19 -t-o the carrier frequency, is inserted in the emitter of T8, thereby introducing feedback at the carrier frequency and preventing the output stage becoming heavily loaded at this frequency. The values of L9 and C19 should be so chosen that the impedance of L9 and C19 is several times the normal input impedance `of T 8 at the carrier frequency. Bias is supplied to the transistor T8 through a resistor R15. The oscillator may be switched on by means of .the switch J.

In the 4transmitters described in lFIGS. 3, 4 and 5, all the transistors may be of a low-rated type, except for the output stages which may have ratings of the order of 100 milliwatts. The tuned transformers may conveniently be wound on ferrite pot cores, eg. 18 mm. diameter, and the ferrite rod aerials L2, L4 and L8 may be about 21A." long X 1A diameter, although greater range may be obtained by using larger aerials where portability requirements permit.

In FIG. 6 the aerial is a loop in series with a ferrite rod L18 and these are together tuned by a ycondenser C28. Current is passed intermittently through contacts N controlled by a mechanically tuned vibrator Mo, such as a tuning fork on tuned reed.

In FIG. 7 the aerial consists of a ferrite rod with a winding L23 -tuned by a condenser C26. Current impulses are fed through part of the winding by a tuning fork Mo which opens and closes contacts N. K is an On-O switch.

In the appended claims, near the loop means within the loop or outside the loop to a distance of not more than twice the maximum width thereof. By pocket transmitter is meant a transmitter which can be put in the pocket or otherwise Worn by a person using the device without causing him inconvenience during normal activity.

l claim:

Apparatus for communication by magnetic induction, comprising a signal transmitting and receiving loopt, a pocket transmitter near said loop, `said transmitter including a transistor oscillator which operates -at a frequency of at least 15 kilocycles, means for modulating the output of said transistor oscillator with an audio signal, a ferrite inductor tune/d to an audio frequency side band, and a filter connected between the output of said modulating means and said ferrite inductor, said filter having a characteristic to reduce the carrier in relation -to saidside band, a receiver connected .to said loop, said receiver including a first resonant circuit connected to the iloop and tuned to the frequency of said transistor oscillator, a second resonant circuit connected to the output of said first resonant circuit and tuned to the frequency of said side band, at least one interference pick-up coil, a phase shifting network connected to said pick-up coil, `an amplier connected to the ouput of said phase shifting network, means connecting the output of said `amplifier to the flrstvresonant circuit and arranged to feed thereto interference picked up by said coil in anti-phase relation to the output from said loop, demodulation means connected to the outputs of said rst resonant circuit and second resonant circuit, and an indicator connected to the output of said demodulation means and arranged to be energized thereby upon detection of a transmitted signal.